1. Field of the Invention
The present invention relates to an image forming method and apparatus for generating a fluid having a predetermined density and/or a predetermined color by changing a mixture proportion of a plurality of inks based on an image signal and leading the thus obtained fluid to an image receiving medium to form an image. Further, the present invention relates to a recording head for use in this image forming apparatus.
2. Description of the Prior Art
U.S. Pat. No. 4,109,282 (which will be referred to as a prior art reference 1, hereinafter) discloses a printer having a structure such that a valve called a flap valve is provided in a flow channel for leading two types of liquid, i.e., clear ink and black ink onto a substrate for forming an image. The flow channel for each ink is opened/closed by displacing this valve so that the two types of liquid are mixed in a desired density to be transferred onto the substrate. This enables printout of an image having the gray scale information which is the same as that of the image information displayed on a TV screen. In this reference is disclosed that a voltage is applied between the flap valve and an electrode provided on a surface opposed to the flap valve and the valve itself is mechanically deformed by the electrostatic attracting force to cause displacement of the valve. Further, the ink is absorbed in paper by a capillary phenomenon between fibers of the print paper.
U.S. Pat. No. 4,614,953 (which will be referred to as a prior art reference 2, hereinafter) discloses a printer head apparatus by which only a desired amount of multiple types of ink having different colors and solvent is led to a third chamber to be mixed therein. In this reference is disclosed that a chamber and a diaphragm-type piezoelectric effect device attached to this chamber are used as means for check-weighing a desired amount of ink and a pressure pulse obtained by driving this piezoelectric device is utilized.
Unexamined Japanese Patent Publication (KOKAI) No. 201024/1993 (which will be referred to as a prior art reference 3, hereinafter) discloses an ink jet print head including: a liquid chamber in which a carrier liquid is filled; ink jet driving means provided in the liquid chamber; a nozzle communicating with the liquid chamber; and a mixing portion for mixing ink to the carrier liquid in this nozzle. In this reference is also disclosed that adjusting means for adjusting an amount of mixture of ink to a desired value is provided.
Similarly, Unexamined Japanese Patent Publication (KOKAI) No. 125259/1995 (which will be referred to as a prior art reference 4, hereinafter) discloses an ink jet recording head including: first and second supplying means for supplying inks having first and second densities, respectively; and controlling means which controls an amount of supply of the second ink by the second supplying means so that a desired ink density can be obtained.
In this reference 4, employment of a micro-pump which has an exclusive heating device and is driven by its heat energy is disclosed. As this micro-pump, there is disclosed an example such that the heat energy is generated by the heating device and a pressure obtained by the nucleate boiling caused due to the heat energy is used to drive, e.g., a piston-type valve or a cantilever-like valve. Further, this reference 4 describes that an inflow of ink can be effectively controlled in an area where an inflow is particularly small by adopting an actuator consisting of shape memory alloy to this valve.
Unexamined Japanese Patent Publication (KOKAI) No. 207664/1991 (which will be referred to as a prior art reference 5, hereinafter) discloses a structure which is similar to that in the prior art reference 2 but does not use a third chamber for mixing a plurality of types of ink.
Unexamined Japanese Patent Publication (KOKAI) No. 156131/1997 (which will be referred to as a prior art reference 6, hereinafter) discloses an ink jet printer comprising a plurality of printer heads for forming an image having multiple colors based on image data. Ink and diluent are mixed to obtain diluted ink which is jetted from a nozzle so that a recording image is formed on a recording medium. The ink jet printer ejects the diluent from at least one printer head out of the multiple printer heads when all-white image data, that is, data representing that amount of mixture of ink is too small to realize a clear printing density, is input. As a result, a rapid change in tone (a tone jump) is prevented and the additional consumption of the diluent is suppressed to improve drying characteristics.
Unexamined Japanese Patent Publication (KOKAI) No. 264372/1998 (which will be referred to as a prior art reference 7, hereinafter) discloses employment of a plurality of line heads in which ink ejection nozzles are linearly aligned. In this example, when the respective line heads are biased and arranged in a direction for feeding print paper and positions of nozzles in the respective line heads are biased relatively to a direction of the width of the print paper, the pixel density can be enhanced. Further, ink having a single color is ejected from each nozzle, and ink droplets having different colors are combined by ejecting ink having different colors in accordance with the line heads, thereby representing predetermined colors on the print paper.
In the prior art disclosed in the prior art reference 1, the ejection ports for two types of liquid are separately formed directly on the print paper, and the respective types of liquid are separately attracted on the print paper by the capillary phenomenon immediately after ejection. Therefore, a quantity of attraction of each liquid on the paper readily fluctuates under the influence of the paper quality of the print paper, which results in the unstable image quality or difficulty of formation of an image having high fidelity to the image signal.
In any of the prior arts disclosed in the prior art references 2 to 6, a plurality of inks are previously mixed or caused to be confluent, and thereafter the mixed liquid (including the confluent liquid) is led onto the print paper. A plurality of the inks are brought into contact with each other in the mixing portion (the confluence portion), and each ink is ejected by a predetermined amount to be mixed. Namely, the ejection port for each ink is formed and assembled in the mixing portion. Each ink can not therefore prevent from being naturally diffused with each other.
For example, even if a given ink is not ejected into a mixing chamber in accordance with the image signal, this ink is naturally diffused in the mixing chamber. Thus, the density and/or color of the finally mixed ink liquid differs from the image signal, and an image which is true to the image signal can not be formed. When the distortion of the contact interface occurs due to a vibration in the mixing portion or any other disturbance even though the natural diffusion of the ink is small, the undesired mixing of ink is facilitated and the above-described problem becomes more prominent.
Since the ink having a single color is ejected from one nozzle in the prior art disclosed in the prior art reference 7, one pixel is formed by multiple (three, four or more colors) ink droplets. Therefore, the pixel density is hard to be enhanced, and improvement of the image quality is also restricted.
In the prior art reference 3 is disclosed that adjusting means functioning as a check valve is provided in the vicinity of the opening of the ink channel formed in the mixing portion in order to mainly prevent the inks from being naturally diffused with each other. However, provision of the adjusting means having the check valve structure complicates the print head configuration and leads to problems such as difficulty in manufacturing, reduction in productivity or increase in the manufacturing cost.
Further, although in the prior art reference 6 is disclosed that the colorless diluent continues to flow in case of all-white image data in order to avoid a rapid change in tone (tone jump), the ink which is not colorless and transparent is continuously diffused in this diluent in this case, and hence the above-mentioned problems can not be prevented.
The present invention has been accomplished under the circumstances as aforementioned, and a first object thereof is to provide an image forming method, wherein, when an ink liquid having a desired density and/or color is generated by mixing inks having multiple different densities and/or colors and this ink liquid is transported to an image receiving medium to form an image, such a problem as that an image having high fidelity to an image signal can not be obtained because the density and/or color of the ink liquid differs from the image signal by mixing at least an image forming ink into the ink liquid whose mixture proportion is set by the image signal by natural diffusion and the like is solved by an extremely simple method, thereby obtaining an image which is true to the image signal.
In addition, it is a second object of the present invention to provide an image forming apparatus used for implementing this method. Moreover, it is a third object of the present invention to provide an recording head used for manufacturing the image forming apparatus.
According to the present invention, the first object can be attained by an image forming method for ejecting a fluid constituted by a plurality of inks from a common ink ejection port while changing a mixture proportion of a plurality of said inks with respect to one pixel based on an image signal and transporting a plurality of said inks to an image receiving medium which is moved relatively to said ink ejection port to form an image; wherein at least one of a plurality of said inks is an image forming ink for substantially forming an image after dried out and an ink flow rate of said image forming ink is controlled in such a manner that a volume flow rate per unit time does not become zero.
A minimum addition amount of the image forming liquid can be equal to or above a flow rate required for refreshing a volume of this image forming ink mixed with any other ink by natural diffusion. However, since the addition amount should be suppressed to such a value as that a change in density and/or color due to addition of this ink does not result in degradation of the image quality, it is preferable to set the addition value in such a manner that a change in optical density of the ink liquid due to addition of this ink is less than 0.1. Here, the optical density means such a degree as that a substance absorbs the light and, when it is assumed that the optical density is represented as D; an intensity of an incident light ray, I0; and an intensity of a transmitted light ray; I, the optical density can be defined by D=log10 (I0/I). It is preferable that vibration absorption is performed at a portion where a plurality of inks becomes confluent can suppress generation of turbulence of the contact interface due to vibration and disturbance of the ink to prevent diffusion.
Print paper may be used as the image receiving medium, and an image can be directly formed on this print paper. However, it is possible to adopt a mode such that a drum-like or belt-like intermediate image receiving medium is provided between the ejection port and the image receiving medium such as a recording sheet and the ink liquid supplied from the ejection port is loaded on the intermediate image receiving medium, so that the ink liquid is then transferred to the image receiving medium. Preferably, the ink ejection ports may be separately provided in accordance with pixels aligned in a direction of the width of the image receiving medium (a direction orthogonal to the moving direction). The ink ejection ports may be formed into a slot-shaped opening which is elongated in a direction of the width of the image receiving medium when changing the density and/or the color only in the moving direction of the image receiving medium.
When it is determined that at least one type of ink is image non-forming ink, i.e., ink which is or becomes transparent and colorless after dried out (which will be referred to as image non-forming ink or clear ink hereinafter), the density can be controlled by changing a proportion or mixing ratio of the image non-forming ink in the ink liquid. It is preferable to add the image non-forming ink to the ink liquid any time so that the amount of supply of the image non-forming ink not become zero. In such a case, when a decoloration preventing agent such as antioxidant, ultraviolet ray absorber or any other component is included in the image non-forming ink in advance, a color degradation preventing property and others can be imparted to an image. A plurality of inks are determined as inks having colors of yellow, magenta and cyan, and changing a mixture proportion of these inks can form a color image.
Controlling flow rates of a plurality of inks can form an image whose density and/or color can vary in both the moving direction and the width direction of the image receiving medium.
A plurality of inks ejected from the ink ejection port may be transported, i.e., jetted on the image receiving medium as droplets by the ink jet mode, but it is also possible transport a plurality of the inks to the image receiving medium as a continuous flow in place of the droplets (the continuous coating mode). In case of this continuous coating mode, a flow of liquid can be ejected or extruded as a continuous flow and transported to the image receiving medium through a slot opening connecting the ink ejection ports provided for the respective pixels in the width direction.
A flow rate of a plurality of inks can be controlled by the various methods. For example, an ink supply pressure with respect to each ink channel can be maintained constant while a cross sectional area of each ink flow channel can be changed by a piezoelectric device. In this case, a diaphragm valve facing to the flow channel is opened/closed by the piezoelectric device. The piezoelectric device can be driven by a mechanical natural frequency (a resonance frequency) of the device itself, and the time period for driving the device is changed by varying a pulse number of this frequency in order to control the flow rate. It is also possible to continuously control a quantity of distortion (an opening of the diaphragm valve) of the piezoelectric device by an analog signal and, in this case, the flow rate is controlled by a voltage of the analog signal.
A flow rate supplied to each ink channel may be controlled by changing a discharged quantity of an ink feed pump. For example, the ink feed pump is driven by a pulse motor (a stepping motor), and the ink flow rate can be controlled by the driving pulse number of this pulse motor. The ink feed pump includes: at least one check valve provided to the ink channel; a cavity provided in the vicinity of this check valve; and a movable member for changing a volumetric capacity of the cavity, so that the pump discharges the ink by changing a volumetric capacity of the cavity. Such pump can be used as an ink feed pump.
The check valve used in the ink feed pump may be constituted by a geometrical form by which a resistance relative to the ink flow direction becomes small and that relative to the reverse direction becomes large. Such a check valve has no movable portion and can be produced by utilizing a method for manufacturing an integrated circuit or a printed wiring board or that for manufacturing a micro-machine. The ink feed pump may be driven by the pulse motor.
When the ink feed pump driven by the pulse motor is provided, the ink feed pump used in this example may preferably be of a volumetric capacity type by which an amount of ejection is proportionate to a quantity of rotation of the motor and, for example, a pump for squeezing a flexible tube appressed against the inner surface of a circular case from the inner peripheral side by an eccentric in a defined direction, a vane pump, a gear pump and others are suitable.
The ink feed pump provided to each ink channel can be formed by the piezoelectric device and the check valve. In this case, the piezoelectric device is a diaphragm valve driven by a mechanical resonance frequency inherent to the device. By controlling the pulse number (pulse number in a defined period of time or a unit time) of the driving frequency of each piezoelectric device, a ejection volume flow rate from each ink channel can be controlled.
According to the present invention, the second object can be attained by an image forming apparatus for ejecting a fluid constituted by a plurality of inks from an ink ejection port while changing a mixture proportion of a plurality of said inks based on an image signal and transporting a plurality of said inks to an image receiving medium which is moved relatively to said ink ejection port to form an image, said image forming apparatus comprising:
ink flow controlling means for independently controlling ink flow rates of a plurality of said inks;
a processor for calculating an ink flow rate of each ink in such a manner a volume flow rate per unit time of at least one image forming ink for substantially forming an image after dried out does not become zero, while maintaining a mixture proportion of each ink corresponding to said image signal; and
a driver for driving said ink flow controlling means based on a result of calculation by said processor.
In order to control the ink flow rate, a diaphragm-type flow control valve driven by a piezoelectric device may be provided to the respective ink channels, for example. In place of the diaphragm valve driven by the piezoelectric device, a diaphragm valve driven by the heat-pressure effect or a counterpart driven the electrostatic attraction force or the electrostatic repulsive force may be used. In such a case, it is needless to say that the ink supply pressure with respect to the ink channel is always maintained constant. Additionally, a discharge quantity of the ink feed pump for supplying ink to the ink channel can be controlled without using the flow control valve. Preferably, such pump is of a volumetric capacity type which is driven by the pulse motor.
The ink flow controlling means may comprises: a check valve provided to the ink channel; a cavity provided in the vicinity of the check valve; and a movable member for changing a capacity of the cavity and have a structure for ejecting the ink by varying a capacity of the cavity. In this example, the check valve may have a geometrical form such that an ink flow resistance with respect to a flow direction of the ink becomes small while the same with respect to the reverse direction becomes large. The movable member can be constituted by a diaphragm driven by the piezoelectric device (or formed by the piezoelectric device itself). The movable member can be made up of a diaphragm driven using the heat-pressure effect, the electrostatic attraction force or the electrostatic repulsive force, the magnetic distortion effect, the interfacial tension effect of a fluid which is different from the ink, and others or a diaphragm driven by air bubbles generated by the electrolytic process of a fluid which is different from the ink.
The ink ejection ports are arranged in accordance with pixels aligned in a direction of the width of the image receiving medium and they are independently opposed to the image receiving medium. In this case, the ink droplets can be transported by the ink jet mode. Additionally, in this case, the ink may be applied by the continuous coating mode in place of the ink jet mode. When using the continuous coating mode, the fluid ejected or extruded from each ink ejection port can be led to the image receiving medium through a slot opening which is elongated in a direction of the width of the image receiving medium. A flow of the ink liquid can be further stabilized as a steady flow to be led to the image receiving medium by using the slot opening in this manner.
In case of the continuous coating mode, the liquid ejected from the ink ejection port may be transported to an intermediate image receiving medium such as a transfer drum, and the ink liquid can be transferred from this intermediate image receiving medium onto a final image receiving medium such as recording or print paper. As described above, the ink liquid ejected from the ink ejection port can be smoothly transferred by using the intermediate image receiving medium, and the deteriorated image quality due to the unequal quality of the image receiving medium such as print paper can be prevented from being generated.
According to the present invention, the third object can be attained by a recording head for use in the above-mentioned image forming apparatus, wherein plural ink ejection ports are arranged on a straight line which is orthogonal or substantially orthogonal to a relative displacement direction of an image receiving medium.
When the adjacent ink ejection ports are distributed to multiple parallel straight lines which are orthogonal or substantially orthogonal to the relative displacement direction of the image receiving medium, the pixel density can be enhanced.
Since a flow rate (volume flow rate per unit time) of at least one image forming ink, which substantially forms an image after dried out, one of a plurality of inks ejected from one ink ejection port is managed so as not to be constantly zero, a mixture amount of this image forming ink can be always grasped and managed. In this case, since a diffusion range or length of the liquid obtained by natural diffusion of the ink with respect to one pixel is considerably short, it is preferable to determine a flow rate required for refreshing a volumetric capacity to the extent of diffusion as a minimum flow rate. As a result, a fluctuation in color and/or density due to natural diffusion of the ink can be suppressed, thereby forming an image having the high image quality.